1. Field of the Invention
This invention relates to an improved CVD process for depositing a metal layer on a semiconductor wafer. More particularly, this invention relates to an improved CVD process for depositing a metal layer on a semiconductor wafer wherein reaction is inhibited between the metal deposition gases and materials already on the wafer under a shadow ring used to inhibit deposition of the metal on the backside and end edge of the wafer.
2. Description of the Related Art
In the deposition of a metal layer, such as a tungsten layer, on the surface of a semiconductor wafer, a problem was previously encountered with undesired deposition of the metal on the backside and end edge of the wafer, due to the migration of the deposition gases, from above the wafer, around the end edge of the wafer to the backside of the wafer.
These deposition gases are usually admitted to the CVD chamber from a gas inlet or "showerhead" located adjacent the top of the CVD reactor, i.e., above the wafer which is conventionally mounted horizontally in the reactor with the upper or front surface, onto which the metal layer is to be deposited, facing toward the incoming deposition gases.
To solve this problem of undesired deposition on the backside of the wafer, it was previously proposed to use a shadow or shield ring which surrounds the wafer and has a central opening slightly smaller than the OD of the wafer so that the shadow ring may be lowered onto the wafer to engage the peripheral edge of the front surface of the wafer, leaving the front surface available for deposition while effectively blocking migration of the deposition gases around the end edge of the wafer to deposit on the backside of the wafer. An opening is further provided in the shadow ring adjacent the end edge of the shadow ring through which a purge gas is upwardly flowed from beneath the shadow ring. This flow of purge gas acts as a further inhibiter or shield to prevent the deposition gases from reaching the backside of the wafer.
While this method and apparatus does inhibit the unwanted deposition of metal materials on the backside of the wafer, its use has created certain other problems, in particular when certain metals are deposited on the semiconductor wafer.
For example, when a layer of tungsten is deposited on a semiconductor wafer, it is common to provide an underlying layer of titanium nitride as an adhesion or "glue" layer which facilitates the bonding of the tungsten metal to the underlying substrate. To further facilitate the deposition of a satisfactory tungsten layer thereon, it is also common to deposit a nucleation layer of tungsten at a low pressure, e.g., of less than 10 Torr, over the titanium nitride layer prior to the main deposit of tungsten, from WF.sub.6 gas at a pressure of about 80 Torr. This nucleation layer is deposited using a reaction of WF.sub.6 with SiH.sub.4.
However, it has been found that when a shadow ring is used to shield the backside of the wafer from deposition of tungsten thereon, some of the WF.sub.6 gas, from the higher pressure main deposition, may migrate under the shadow ring. This can cause problems because materials in some forms of titanium nitride, e.g., unreacted titanium, may react with the WF.sub.6 gas, resulting in an undesirable peeling of one or both of the tungsten/titanium nitride layers.
This problem does not occur in the main (unshielded) portion of the front surface of the wafer because the low pressure nucleation layer of tungsten, applied prior to the main deposition, further acts as a barrier layer through which the WF.sub.6 gas from the main deposition cannot penetrate to reach the underlying titanium nitride. This tungsten barrier layer is formed without reaction between the titanium nitride layer and the WF.sub.6 gas used to form the tungsten nucleation layer because the deposition of the tungsten nucleation layer is carried out using a mixture of WF.sub.6 and SiH.sub.4 gases which react very quickly to form tungsten so that little, if any, unreacted WF.sub.6 gas is available to penetrate into the underlying titanium nitride layer.
However, since the tungsten nucleation layer is deposited at a lower pressure than the main tungsten deposition, the gases forming the tungsten nucleation layer do not migrate under the shadow ring as easily as do the subsequent higher pressure tungsten deposition gases. The result is that the WF.sub.6 gas flowing into the reaction chamber during the main tungsten deposition can migrate under the shadow ring and then penetrate into the underlying (and unprotected) titanium nitride layer on the front surface of the wafer beneath the shadow ring, resulting in the undesired peeling and particle formation.
It should be further noted that not all of the front surface of the wafer beneath the shadow ring at the periphery of the wafer is covered by titanium nitride. There are also small areas of exposed oxide representing regions where wafer clamps contacted the front surface of the wafer during the titanium nitride formation. Therefore, even if one could somehow protect the wafer from backside deposition in some other manner, i.e., eliminating the shadow ring, deposition of a thick (e.g., 1-2 micron) layer of tungsten over the oxide portions would also result in undesirable peeling of the tungsten layer at those points.
It would, therefore, be desirable to provide a process of forming a metal layer, such as a tungsten layer, over a semiconductor wafer in a manner which would protect the backside of the wafer from deposition, yet not result in an undesirable peeling of material deposited on the front side of the wafer beneath the shadow ring.